Known methods of marking a video signal are disclosed in WO 93/00769 and EP-A 0 518 616. The known methods modify a video signal such that certain disturbances in the picture are introduced upon playback. It is also known to mark a video signal by adding data to the signal. One method is to accommodate data in the vertical blanking interval. Another method is to blank a rectangular picture portion and replace said picture portion by a sequence of white and black spots that can be detected by a photodiode in front of the picture tube.
Nowadays, the subject of marking a signal is especially relevant with respect to the copy protection of digital video discs. It is desirable to provide the signal with a detectable code ("watermark") so as to identify the signal as being authentic and to render reproduction of the signal possible under specified conditions only. Video disc players are envisaged that are adapted to playback authentic video discs only. They reproduce recorded program material only if the video signal's watermark corresponds with a code ("wobble key") physically embedded in a given groove of the disc and not provided on a recordable disc.
The prior art methods mentioned above are not suitable to fulfil this task. They rely on the different sync behavior of different types of domestic equipment, render it possible for a consumer to delete the watermark, or suffer from the disadvantage that the embedded mark affects the displayed signal.